Wastewater effluents originating from metallurgical (e.g. electroplating, mining, metal finishing, etc.) and process (e.g. printing) industries contain heavy metals, such as arsenic, cadmium, copper, gold, nickel, and zinc. These effluents must therefore be adequately treated before being discharged into the water bodies to minimize the effect of the contaminating heavy metals on the environment, especially with regard to the potable water supply. Treatment methods for metal removal from wastewater include precipitation, ion exchange, reverse osmosis, electrolysis, and electrodialysis. With the exception of electrolysis, all of these methods generate sludge or concentrated streams which need further treatment. Electrolysis is capable of removing toxic metal ions, through electrodeposition of the metal ion in metallic form at the cathode, and is a very well studied technique. Review articles and cell design are given by Kuhn and Houghton1, Robertson and Leudolph2, O'Keefee and Ettel3 and Weinginger4. The electrolytic process is mainly mass transfer controlled, and cell configuration that increases the mass transfer at the cathode improves the performance of the cell. For example, rotary drum5, fluidized bed6, flow through a porous electrode7, gas-sparging cell8, rotating cylinder electrode9, bipolar trickle cell10, rotating disc11, and tumbling barrel12-16 have demonstrated improved performance.
Despite the improved mass transfer shown in conventional systems, the electrolytic method becomes expensive when the concentration of the wastewater becomes low due to increased ohmic resistance. Under these conditions, hydrolysis becomes the dominant reaction, manifesting itself as a low current efficiency for metal deposition. One way of solving this problem is to add salt to increase the electrolyte conductivity. However, this results in an undesirable increase in the total dissolved salts in wastewater.
Electrodialysis uses a number of anion and cation exchange membranes held between two electrodes. This technique is capable of treating low concentration wastewater, but produces a concentrated stream that needs further treatment. The first patent on electrodialysis was awarded in 197617. Electrodialysis has been used to treat seawater18,19, to produce chemicals20-22, to recover metal ions in the metal finishing and metallurgical industry23-26, and to treat industrial wastewater27-35.
It is attractive to integrate electrolytic and electrodialytic processes to treat wastewater of moderate concentration and to recover metals. One way of doing so is shown in FIG. 1, which shows a general concept for treating wastewater of moderate concentration and for recovering metal using integrated electrolytic and electrodialytic processes. The wastewater stream is fed to the electrodialytic cell in countercurrent fashion. As the wastewater passes through the cell, the cations (metal ions) and anions diffuse through the cation and anion exchange membranes, respectively. Consequently, one stream loses metal ions to the other.
The diluted stream is taken out as the treated wastewater, while the concentrated stream is sent to the electrolytic cell. The flow rate and area of exchange are designed so that the exiting concentrated stream has enough concentration so that it is treated easily in the electrolytic cell where the metal is recovered on the cathode. Optimal flow rate and area of exchange are easily calculated by a person of ordinary skill in the art. The residence time in the electrolytic cell is such that the concentration of the exiting stream is similar to the original wastewater and this stream is recycled to the electrodialytic cell. Optimal residence times are easily calculated by a person of ordinary skill in the art. The overall system is complex and the wastewater needs to be pumped back and forth between the two cells. Pumping will increase the operating cost of the system.
Accordingly, there remains a need for an efficient and cost effective apparatus and process for removing and recovering metal ions from wastewater streams using electrochemical methods, which overcome the aforementioned disadvantages.